ARM: OMAP2+: Fix booting with configs that don't have MFD_SYSCON

With the recent changes omaps have developed a dependency to MFD_SYSCON.
This is used for system control module generic register area and some
clocks.

We do have it selected in omap2plus_defconfig, but targeted config
files may not have it selected. Let's make sure it's selected like
few other ARM platforms are already doing.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>